fields_for not working outside ERB

If one calls fields_for in a helper method, then the block result is not returned by the method. For example.

# application_helper.rb
def foobar
  fields_for(:object, Object.new) do |f|
    "foobar"
  end
end

Calling <%= foobar %> in the view will not output "foobar" in Rails 3 Beta 2 but it will in Rails 2. This breaks some nested form solutions.

I'll look into submitting a patch, but I think changing with_output_buffer to capture in the fields_for method will do the job.